This invention is directed to lubricating fluids that contain elemental sulfur. More particularly, it is directed to the preparation of cutting fluids containing elemental sulfur and to stable sulfur suspensions that are employed in said cutting fluids.
It is well known that elemental sulfur may be added to various lubricating oils and that such sulfur provides the oil with high-load carrying and antiwear properties. Elemental sulfur is quite beneficial in cutting oils, which are used in the machining, cutting, and forming of metals and metal parts. The sulfur provides a smoother surface finish to the metal being worked upon and extends the life of the cutting tools that are being used to machine the metal parts.
While it is conceivable that one or more sulfur compounds other than elemental sulfur could be used to provide the same sort of antiwear and lubricative properties, the use of elemental sulfur results in a lower cost. In addition, active sulfur compounds usually furnish a chemical activity of sulfur which is less than that of elemental sulfur.
In U.S. Pat. No. 3,309,315, Tarazi, et al., treated a naphthenic base crude oil with a moderate sulfuric acid treatment, which treatment enabled the remaining unsaturated components of the oil to be more susceptible to a subsequent reaction with certain sulfur compounds so as to form beneficial, solution-stable, sulfur-bearing, organic compounds in the oil. In U.S. Pat. No. 3,252,908, Coleman considered an oil-soluble additive composition comprising about 5 wt% to about 90 wt% of an oil-soluble acylated amine, about 90 wt% to about 5 wt% of a phosphatide, and about 0.01 wt% to about 48 wt% of a mineral oil, which composition stabilizes the suspension of elemental sulfur in a lubricating oil. In U.S. Pat. No. 4,073,736, Schick, et al., employed an antiwear amount of polypropylene glycol and sulfur, and a solubility improving amount of a monohydric alcohol having from 5 to 30 carbon atoms, which lubricant additive is especially useful in metal working and cutting applications.
The solubility of elemental sulfur in a base oil is approximately 0.5 wt%. Hence, many cutting oils are made by dissolving up to 0.5 wt% sulfur in the base oil. Molten sulfur can be used and is pumped directly into the oil, but at a temperature of about 116.degree. C. (240.degree. F.) to 121.degree. C. (250.degree. F.), the temperature range that includes the melting point of sulfur. The oil must be heated to this temperature when the molten sulfur is introduced into it. If the temperature of the oil were below the melting point of sulfur, agglomerates of solid sulfur would form before the molten sulfur was able to be fully dissolved in and dispersed throughout the oil. Since large sulfur particles dissolve very slowly, the addition of molten sulfur to a base oil at a temperature much below 116.degree. C. (240.degree. F.) is not feasible on a commercial basis. For a like reason, the addition of solid sulfur to the base oil at such a temperature would not be feasible on a commercial basis.
There have now been found a process for preparing a stable sulfur suspension and a process for preparing a sulfurized cutting fluid in which elemental sulfur can be effectively added to the cutting fluid at a temperature as low as 54.degree. C. (130.degree. F.), or even lower. The process for preparing the sulfur suspension permits sulfur to be added to a suspending medium at room temperature, while the process for preparing a sulfurized cutting fluid permits the sulfur to be introduced into the fluid at temperatures that are as low as 54.degree. C. (130.degree. F.), or even lower, and to dissolve rapidly into the fluid without the formation of a significant amount of solid sulfur agglomerates.